1. Field of the Invention
The present invention relates to a density sensing device, and a fuel cell system having the density sensing device. The present invention, in particular, includes a fuel cell system with a density sensing device that includes a cantilever density sensor.
2. Description of the Related Art
A fuel cell is a power generation system that produces electric energy by electro-chemical reaction between hydrogen and oxygen. Hydrogen is contained in hydrocarbon-based substance such as methanol, ethanol, and natural gas. Oxygen is contained in air.
Depending on types of electrolyte, fuel cells can be categorized into a phosphoric acid fuel cell, a molten carbonate fuel cell, a solid oxide fuel cell, a polymer electrolyte fuel cell, and an alkaline fuel cell, etc. These fuel cells are basically operated by the same principle, but are different in terms of fuel, operation temperature, catalyzer, and electrolyte.
A polymer electrolyte membrane fuel cell (PEMFC) has advantages of a remarkably high output characteristics, low operation temperature characteristics, and rapid starting and responding characteristics over other types of fuel cells. PEMFC is widely used in a mobile power source of portable electronic equipments, in a transportable power source such as a power source for automobile, and in a distributed power source such as a stationary power plant that can be installed in a house or a public building.
A direct methanol fuel cell (DMFC) is similar to the polymer electrolyte membrane fuel cell, but is capable of directly supplying a liquid methanol fuel to a fuel stack. The direct methanol fuel cell is more advantageous in terms of size, because it does not require a reformer, which is required in the polymer electrolyte membrane type fuel cell to obtain hydrogen from fuel.
The direct methanol fuel cell includes a stack, a fuel tank, and a fuel pump. The stack generates electric energy through electrochemical reaction from a fuel, which contains hydrogen and oxidizer such as oxygen.
The stack has a stacked structure of several to several tens unit fuel cells, each of which is typically configured of a membrane electrode assembly (MEA) and a separator. The membrane electrode assembly includes an anode electrode (fuel electrode or oxidation electrode), a cathode electrode (air electrode or reduction electrode), and a polymer electrolyte membrane disposed between the anode electrode and the cathode electrode.
In the fuel cell such as a direct methanol fuel cell in which fuel is supplied to a stack in liquid state, operation efficiency greatly depends on mole density of fuel supplied to an anode electrode and a cathode electrode. For example, if the mole density of the fuel supplied to the anode electrode is high, the amount of the fuel flowing from the anode side to the cathode side increases. Therefore, counter electromotive force is generated due to the fuel reacting on the cathode electrode, so that output voltage decreases. The fuel cell stack has the best operation efficiency at an optimized fuel density. Therefore, a method to properly control a mole density of fuel in the direct methanol fuel cell system has been required for the efficient operation of the fuel cell.
Therefore, the direct methanol fuel cell system or the other type fuel cell systems can include a device for measuring density of a fuel solution stored in units such as a stack, a fuel tank, a recycle tank, or pipes between any two units of the fuel cell system.
In the fuel cell, the operating state of the fuel cell system can be monitored by measuring the density of aqueous fuel solution, and the operation efficiency of the fuel cell can be improved by optimizing condition of each unit forming the fuel cell system according to the monitored result.
Also, in the fuel cell, in which hydrogen is supplied to an anode, such as the polymer electrolyte membrane type fuel cell, there can be a material in a liquid form such as condensate of material released from a cathode side. A density measurement of the liquid material can be provided if necessary.
Therefore, in the fuel cell, the density measurement of solution is important to improve the operation efficiency of the fuel cell. However, in order to install a measurement device for measuring the density of a solution, the density measurement device should satisfy many requirements, such as a small size, an accurate density measurement, a fast density detection, and low cost.
In order to meet the requirements, density sensors, such as a polymer adsorptive density sensor, an ultrasonic type density sensor, and a resistance measurement type density sensor have been proposed. These density sensors, however, do not sufficiently satisfy all the requirements. Therefore, it has been difficult to build a fuel cell system that has high performance and can be manufactured at low cost.
Also, in the case of a small size density sensor, if solid impurities are included in a solution, there is a high possibility that an error would occur in the density measurement due to the presence of the impurities. In particular, in a fuel cell system of a direct methanol fuel cell (DMFC), because there is a very high possibility that the impurities can be included in the fuel while passing through a fuel mixing unit, a device to solve this problem has been required.
Most of the density sensors that have been developed up to now greatly depend on the temperature of the solution to be measured. In order to offset the deviation due to the temperature, two methods have been proposed. One method is to include a separate temperature sensor to measure temperature and a conversion table that shows temperature dependence of the density. Another method is to include a device that keeps the temperature of the solution constant. The first method, however, has problems that the construction of an accurate conversion table is not practical and the adjustment of the conversion table depending on the type of the sensor is not practical. The second method has problems that cost for keeping the temperature of the solution container such as a fuel tank constant is very high.